The World Health Organization (“WHO”) estimates that as much as one-third of the world's population is infected with tuberculosis. In 1998, the latest year for which estimates are available, Mycobacterium tuberculosis (“MTb”) infected 7.25 million people and resulted in 2.9 million fatalities (Farmer, P. et al., Int J Tuberc Lung Dis 2:869 (1998)). Underlying these statistics is an emerging epidemic of multiple drug-resistant (“MDR”) tuberculosis that severely undermines control efforts and is transmitted indiscriminately across national borders (Viskum, K. et al., Int J Tuberc Lung Dis 1:299 (1997); Bass, J. B. et al., Am J Respir Crit Care Med 149:1359 (1994)). Resistance to any of the front-line drugs generally bodes poorly for the patient, who then is committed to a regimen of less active “second-line” therapies. Where multidrug resistance is suspected, the WHO recommends that three or more drugs be administered at the same time, to decrease the chance that the organism will be able to develop resistance to all of the agents.
One of the most efficacious of the second-line drugs is the thioamide ethionamide (ETA) (Farmer, P. et al., supra). Like the front-line drug, isoniazid (INH), ETA is specific for mycobacteria and is thought to exert a toxic effect on mycolic acid constituents of the cell wall of the bacillus (Rist, N. Adv Tuberc Res 10:69 (1960); Banerjee, A. et al., Science 263:227 (1994)). Current tuberculosis therapies include a large number of “prodrugs” that must be metabolically activated to manifest their toxicity upon specific cellular targets (Barry, C. B., III et al., Biochem Pharm 59:221 (2000)). The best characterized example of this is the activation of INH by the catalase-peroxidase KatG, generating a reactive form that then inactivates enzymes involved in mycolic acid biosynthesis (Slayden, R. A. et al., Microbes and Infection (2000) (in press); Heym, B. et al., Tubercle Lung Dis 79:191 (1999)). The majority of clinically observed INH resistance is associated with the loss of this activating ability by the bacillus (Musser, J. M., Clin Microbiol Rev 8:496 (1995)), but such strains typically retain their sensitivity toward ETA, suggesting that ETA activation requires a different enzyme than KatG (Rist, N., Adv. Tub. Res. 10, 69 (1960)).
In a striking achievement of molecular biology and genetics, the entire genome of a paradigm M. tuberculosis strain, H37Rv (EMBL/GenBank/DDBJ entry AL123456), was sequenced and published in 1998. (Cole, S. et al., Nature 393,537 (1998)). The genome was found to comprise 4,411,531 base pairs, comprising 3,974 putative genes, of which 3,924 were predicted to encode proteins. Each of the putative genes was accorded a number based on its position in the genome relative to a selected start site. The function of many of the putative genes, however, could not be determined when the genome was sequenced and published, and their function remains unknown today.